Field of the Invention
The following invention relates to a crystal oscillator circuit.
Crystal oscillators are electrical systems that can oscillate with a better frequency constancy than LC oscillators. In this case, crystals mechanically oscillate by being stimulated by electrical fields. An oscillating crystal having connected electrodes electrically behaves like a high Q-factor resonant circuit.
A so-called Pierce oscillator, which is representative of fundamental frequency oscillators, has two capacitors forming a series resonant circuit that is stimulated by an amplifier. In this case, a complementary metal oxide semiconductor (CMOS) inverter is used, for example, as the amplifier. A Pierce oscillator such as this is specified, for example, in the Tietze, Schenk: Halbleiter-Schaltungstechnik, [Semiconductor circuit technology] 10th Edition, 1993, on pages 466-469. In the Pierce oscillator, one of the two capacitors is designed to be adjustable. The oscillation frequency of the resonant circuit is influenced by adjusting the series capacitor. However, frequency adjustment using capacitors can also be carried out, for example, by connecting or disconnecting capacitor elements in discrete steps.
In crystal oscillators such as these, in which frequency adjustment is carried out using capacitors that are connected to ground, it is possible for the operating point to be shifted so far that the phase reserve of the system that can oscillate is no longer adequate, and the system becomes unstable. Instability such as this is disadvantageous in particular in the case of oscillators that have amplitude regulation.
It is accordingly an object of the invention to provide a crystal oscillator circuit having an improved stability response.
With the foregoing and other objects in view there is provided, in accordance with the invention a crystal oscillator circuit including: an oscillating crystal having a first and a second connection; a quick-response amplifier having an input connected to the first connection of the oscillating crystal and an output connected to the second connection of the oscillating crystal; a device for operating point adjustment, the device coupling the oscillating crystal to the input of the quick-response amplifier; a reference node; a first capacitive component for connecting the first connection of the oscillating crystal to the reference node; a second capacitive component for connecting the second connection of the oscillating crystal to the reference node; and a compensation capacitor connected to the input of the quick-response amplifier and to the reference node. The first capacitive component and/or the second capacitive component is adjustable. The compensation capacitor is adjustable in a complementary manner to the first capacitive component and the second capacitive component.
In accordance with an added feature of the invention, the first capacitive component includes a plurality of capacitive components connected in parallel; the second capacitive component includes a plurality of capacitive components connected in parallel; the plurality of the capacitive components of the first capacitive component and the plurality of the capacitive components of the second capacitive component are connectable and disconnectable in pairs independently of one another; the compensation capacitor includes a plurality of capacitive components connected in parallel; and each one of the plurality of the capacitive components of the first capacitive component is associated with a respective one of the plurality of the capacitive components of the compensation capacitor.
In accordance with an additional feature of the invention, each one of the plurality of the capacitive components of the first capacitive component and the respective one of the plurality of the capacitive components of the compensation capacitor associated therewith has a fixed capacitance ratio.
In accordance with another feature of the invention, the plurality of the capacitive components of the compensation capacitor can be connected and disconnected independently of one another.
In accordance with a further feature of the invention, there is provided, a plurality of switches; and a drive circuit. Each one of the plurality of the capacitive components of the first capacitive component, the plurality of the capacitive components of the second capacitive component, and the plurality of the capacitive components of the compensation capacitor are coupled to a respective one of the plurality of the switches enabling connection and disconnection thereof. Each one of the plurality of the switches has a control connection. The drive circuit supplies control signals in a non-inverted form to the control connections of ones of the plurality of the switches connected to the plurality of the capacitive components of the first capacitive component. The drive circuit supplies control signals in a non-inverted form to the control connections of ones of the plurality of the switches connected to the plurality of the capacitive components of the second capacitive component. The drive circuit supplies control signals in an inverted form to the control connections of ones of the plurality of the switches connected to the plurality of the capacitive components of the compensation capacitor.
In accordance with a further added feature of the invention, the first capacitive component has a capacitance value; the compensation capacitor has a capacitance value; and a quotient of the capacitance value of the first capacitive component and the capacitance value of the compensation capacitor is greater than 5.
In accordance with another added feature of the invention, a minimum value capacitor permanently connects the input of the quick-response amplifier to the reference node.
In accordance with yet an added feature of the invention, the quick-response amplifier is an inverter.
In accordance with yet an additional feature of the invention, the quick-response amplifier is a CMOS inverter.
In accordance with yet another feature of the invention, the device for operating point adjustment is a resistor.
According to one development of the Pierce oscillator and in the case of the described principle, the oscillation frequency of the oscillator is adjusted by connecting or disconnecting the first and second capacitive component. The introduction of a compensation capacitor makes it possible for the oscillator circuit to maintain a stable response. The compensation capacitor is decoupled from the actual resonant circuit by the device for operating point adjustment, so that the connection or disconnection of the compensation capacitor admittedly has the desired effect on the operating point of the quick-response amplifier, but not on the actual oscillation frequency of the crystal oscillator circuit.
The compensation capacitor is adjusted opposite and in mirror-image form to the first and second capacitive component. This means that the compensation capacitor is disconnected when the first and second capacitive component is connected, and vice versa. With a suitable design, this results in the maintenance of the operating point of the quick-response amplifier. The compensation capacitor accordingly affects only the dynamic operating point of the quick-response amplifier.
According to one advantageous development of the invention, a number of first and a number of second capacitive components are provided. In this case, the number of first capacitive components are connected in parallel with one another. The number of second capacitive components are likewise connected in parallel with one another.
Since the first and second capacitive components can each be connected and disconnected, the parallel circuits preferably each include a series circuit of a switch and of one of the number of first or second capacitive components.
The number of first and the number of second capacitive components are preferably provided in pairs. In this case, one first and one second capacitive component are in each case connected or disconnected at the same time.
The large number of capacitive components allows the frequency at which the oscillating crystal is oscillating to be adjusted in any desired number of steps.
In this case, each pair including a first and a second capacitive component preferably has a respectively associated compensation capacitor. The compensation capacitor in each case has a fixed capacitance ratio with respect to the associated first and second capacitive component, in this case. According to the present principle, in each case one pair of first and second capacitive components are switched at the same time at the associated compensation capacitor, but in the opposite sense and in mirror-image form. This means that, when one pair including a first and a second capacitive component is connected, the compensation capacitor which is associated with this pair and which has a fixed capacitance ratio with respect to the two components is disconnected. With a suitable design, the proposed principle makes it possible first to ensure a simple layout of a crystal oscillator circuit, and second to achieve an exact compensation for the dynamic operating point of the quick-response amplifier.
In order to make it possible to switch associated capacitors in the opposite sense and in mirror-image form, a drive circuit is preferably provided which supplies a control signal in a non-inverted form to the control connections of the switches which are respectively associated with the first and second capacitive components. The switch that is connected to the compensation capacitor that is associated with the pair of first and second capacitive components is supplied with this control signal in inverted form at its control connection. This results in the opposite switching, in mirror-image form, on which the present principle is based, of the compensation capacitor with regard to the first and second capacitive components.
The capacitor values of the first and second capacitive components are preferably very much greater than the capacitive value of their associated, compensating capacitive component. The capacitor values of the first and second capacitive component, which jointly form a pair, are in this case preferably the same. The quotient of the capacitor value of the first capacitive component and the capacitor value of the associated compensation capacitor is preferably greater than 5.
As an improvement, this ensures that the connection or disconnection of compensation capacitors influences only the dynamic operating point of the quick-response amplifier, but not the oscillation frequency of the crystal oscillator itself.
According a further advantageous embodiment of the present invention, a minimum value capacitor is provided, which permanently connects the input of the quick-response amplifier to the reference node.
Together with the device for operating point adjustment, which is preferably in the form of a resistor, the minimum value capacitor forms an RC element, which has low-pass filter characteristics. A low-pass filter such as this advantageously assists the stability of the crystal oscillator circuit.
The described crystal oscillator circuit is preferably constructed as an integrated circuit using complementary metal oxide semiconductor (CMOS) circuit technology.
The quick-response amplifier is preferably formed as a CMOS inverter.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a crystal oscillator circuit, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.